The transfer of electrons constitutes a key reaction in biological electron transport and energy coupling. The reaction is best studied in isolated mitochondria and in submitochondrial particles derived therefrom. The possibility of studying this reaction with modified cytochrome c as electron donors and, as electron acceptors, a variety of intermediate compounds of oxygen with cytochrome a3 and its associated copper or with cytochrome a itself affords different electron donor-acceptor combinations in which electron tunneling processes may be investigated. Electron transport has been found to occur by a nonadiabatic nuclear assisted electron tunneling process optimally studied in the temperature range -30 to -80 degrees. In order to better characterize the spin and valence states of these electron acceptors their properties will be investigated by optical spectroscopy, by magneto circular dichroism and by EDGE and EXAFS X-ray absorption. Charge separation across the membrane is to be investigated by kinetic studies at low temperatures. Biological variation of cytochrome oxidase is to be studied in a variety of genetic variants and in a remarkable strain of Candida utilis and in cytochrome o. The existence of unknown types of cytochrome oxidase is to be further explored by the photochemical action spectrum apparatus activated through a liquid dye laser. The kinetic data will be evaluated by computer simulation study which provides best fit values. The information is applicable directly to in vivo studies of mitochondria in situ as observed by the application of optical techniques to animal models of normal and diseased states.